1. Field of the Invention
This invention relates to ethylene polymer based compositions which are used as shielding materials for electrical conductors which are used in high voltage wire and cable.
2. Description of the Prior Art
Electric wire and cable that is designed to carry electric current under a high voltage, i.e., .gtoreq. 2K volts, usually comprises a core conductor which comprises one or more strands of a conducting metal or alloy such as copper or aluminum, a layer of a semi-conducting conductor shielding, a layer of insulation and a layer of semi-conducting insulation shield. A plurality of neutral wires which are usually made of tinned copped are embedded in or wrapped around the layer of semi-conducting insulation shield. These neutral wires form a concentric ring around the core of the cable.
These semi-conducting shielding compositions have been formed from ethylene polymers and conductive carbon black. However, the present service demands on these high voltage power cables requires the use therein, as the semiconducting insulation shield compositions, of insulation shielding compositions which are adaptable to function at the relatively high temperatures, of the order of 90.degree. to 130.degree.C., to which these cables can heat up to during the transmission of the electrical power through the cable. Unless these insulation shielding materials maintain their physical integrity at such high operating temperatures the concentric neutral wires which are embedded in or wrapped around the insulation shield are subject to penetrating the insulation shielding if the insulation shield experiences any significant amount of deformation as a result of external forces under these elevated use conditions. The ethylene polymer based compositions which have been filled with conductive carbon black and which have been used as the insulation shield for high voltage cables in the past tend to soften and deform precipitously at temperatures of the order of .gtoreq. 70.degree. to 90.degree.C. due to fusion or melting of the ethylene polymer therein. Although a number of solutions to the problem have been proposed, not all of such solutions have provided an insulation shielding composition which will resist deformation at elevated temperatures, and which can be readily fabricated at elevated temperatures, and which will not suffer a concomitant deterioration in embrittlement temperature. These high voltage power cables are used in many instances outdoors, and above ground, in freezing and subfreezing temperatures, and the insulation shielding compositions, therefore, must also resist embrittlement under such external low temperature conditions.
Prior to the present invention, therefore, it has not be readily possible to provide semi-conducting insulation shielding compositions for high voltage power cables which can be readily processed at temperatures of the order of about 125.degree. to 250.degree.C., which will not suffer embrittlement at external temperatures of about 0.degree. to -45.degree.C. and which will resist deformation under internal in-service temperature conditions of about .gtoreq. 100.degree.-120.degree.C.